The upper half of the dialog lets you set a quality level for JPG images. Move the slider control to the desired setting. Lower settings take less space, but deliver less quality. Higher settings are better, but take more file space. The default setting is 80, which is fine for most images.
Check the grayscale option to save the image as that. Please remember if you activate this option! Click the Save as progressive JPG box (like many images from internet) to activate that option. Click either again to remove the choice.
You can also control the saving of original EXIF/IPTC/Comment informations into the new JPG file.
OK. Reading some of the links from Technotiger's Google results confirms that only .gif and .png images support transparent backgrounds. So that part of my memory was correct. But I still can't find the right menu option in Irfanview even though I've got the image in .gif format.
I need transparent background so that I can place the image over another in a publication, and have the original image show through.
Recap, my recollection, and Google's search results, say that .jpg doesn't support transparent background, so I don't think your solution is what I'm looking for, thanks :-)