Smoothing factor does what the name implies, it smoothes the image somewhat. This smoothing isn't enough to be noticable on most images, but may be noticable on something like fractals. A high smoothing factor will reduce filesize but will make the image more lossy/lower visual quality. Since quality is your highest concern with the fractal pics, I would suggest putting it at 0. For other images where fine detail is less important, you may want to go up to 10 to decrease the filesize.
To see the difference for yourself, drag the image in the preview window to a detailed section of the image & try the different settings. You should also hit the + magnifying glass a few times to zoom in so that you can see the results better.
DCT method swaps out speed for size, but leaves quality intact. It doesn't make a huge difference in either size nor compression speed on most images, so it doesn't matter much what setting you choose.
You should have the huffman table option checked for all pics. It decreases the filesize more, without affecting quality. It might add a little time to the compression, but if so it's so little that you can't even measure it.
The progressive option should also be checked for most pics, but will backfire slightly on a few pics. In practice this has the same general effect as the huffman table one, in reduces the size of most pics without affecting quality. Once in a while though, it will actually INcrease the size slightly. This is rare though & doesn't make a major difference, so it's safe to leave checked all the time.
The primary purpose of the progressive option has to do with how the image downloads/displays on a slow connection. Have you ever noticed how some pics will load at perfect quality, from the top to the bottom, while others will load a full pic at low quality & then build up the quality as the pic downloads? Choosing the "progressive" option means that that pic will download at low quality & build up the quality as the pic loads. The reduction in filesize is an unintended (but useful) side effect of this.
Neither of these options will have a huge impact on filesize, but every little bit helps (especially if it doesn't hinder quality).
Seems my JPGs in the fractals folder would actually become heavier files
That's probably because they were previously saved with lower quality settings (for example, the low quality subsampling option is default in most image viewers/editors. In fact, many don't even give you the option of changing it, xnview is one of the few that does.)
As Xntriq said, saving those files as higher quality JPGs would do no good, they would still retain their current data loss but be bigger files. If you can recreate them or have them as BMPs, then you could create new, high quality JPGs for them. The filesize of these new JPGs will still be larger than your current JPGs for them, BUT they will be much higher quality than your current JPGs, making it well worth the tradeoff.
Xntriq:
PngOptimizer is not a plug-in, but a standalone program with batch processing capability.
*shrug* Hard to keep track of which ones are purely stand-alone progs, which ones are both stand-alone & plugins, and which are just plugins. My underlying point remains the same though, that the results can be achieved with xnview alone, and that it's much quicker than most (all?) PNG optimizing programs and plugins, which tend to use brute force & a great deal of processor cycles to shave just a few bytes off the pic.
Speaking of batch processing:
artistgrrl- If you have a lot of BMPs to convert, you can use xnview to batch convert instead of 1 by 1. All the important quality settings are available for plain saving & for batch converting (minus the preview window), they just take an extra step or two to get to.
To batch convert, open xnview, then go tools->batch processing, select your files, choose JPG as the output, then click the options button, and select the settings we already discussed.
You will find a few new options here;
The estimated quality one is irrelevant unless you started out with JPGs.
Rebuild embedded thumbnail is probably also irrelevant unless you started with photos from a camera. You should always have that option marked just in case. (If you don't & your original image had an embedded thumbnail, then sometimes the resulting image will still be bloated because it has a huge thumbnail. In experiments, I've wound up with images where the thumbanil was actuall significantly larger than the final image.)
The 4 "keep [...]" options; If you are working with images from a camera, you'll probably want these checked. If working with images you created from a program, you'll probably want to UNcheck them. All 4 of these are about non-image data. The EXIF and IPTC ones contain data about when the pics were created, what camera settings were used, etc. This info will probably be useful to a dedicated photographer, though it is altogether meaningless to most of us. The ICC profile one has to do with how the colors in your image will be displayed on different computers. Support for this is lacking/incomplete/buggy in most browsers, so it's kinda useless. I'm not certain exactly what the XMP data is.
UNchecking any of these 4 options will remove the respective data from your image PERMANENTLY. It will usually reduce filesize somewhat, but usually not a lot. If you are not certain, you should go ahead and have all 4 of these options CHECKED. this is the safest option.
If you are using xnview's browser mode, you can select all the images you want to convert before going into tools->batch processing, this will avoid having to select the files via the "add files" option in the batch processing window.
And here I was beginning to feel a bit silly asking so many N00B questions. But how ya gonna learn without asking a lot of questions? Sometimes it seems any FAQ section is "above my head" anyway, in some respects...sometimes I just need more detail.
