When you change anything about an image in Irfanview, I suggest you use the Save As option.
You should also be able to manipulate the image in irfanview, just click on Image drop down menu. Options include: increase/decrease colour depth, covert to greyscale, sharpen etc. as well as resize/resample.
Again, use Save As when done.
BMP Microsoft bitmap image format. This is a fairly simple bitmap format that can be viewed on many different platforms but is directly supported by very few browsers. It is not used too much on the web (fortunately, as it is very inefficient) with GIFs, JPEGs, PNGs, and even X-bitmaps being preferred as they are much more portable.
JPEG stands for Joint Photographic Experts Group. JPEG is a color image compression technique that reduces file sizes by eliminating redundant or unnecessary image data. Many digital cameras automatically save images using the JPEG format. JPEG and GIF (Graphics Interchange Format), another compression method, are the most commonly used image file formats.
I don't use a paint program, so I will leave it to others to suggest.