A suggestion: when using jpeg keep both size multiple of 8. Jpeg compression works on 8x8 blocks, so if you have 170x170, it is about the same of 176x176. So if you want little file it's better 168x168.
Eventually yuo can crop some pixels if you want the same aspect ratio. I use imagemagick to resize my images/crop, it is a commandline tool. Irfanview has a small bug when resizing very big images (about 6-7000Pixel in one side), it creates a small black line in the right size of the image.
Also the contrast adjuster of IView is not so good (i had to write my own one).

I also think that 512KB limits is very low, but "a caval donato non si guarda in bocca" (Italian popular phrase about free stuff), so i cannot complain about something free.
I don't use strange effects, multilayer, texture, etc.... my fractal are only about details and math and some of them lose ALL their beauty when resized to 500K.