How to reduce jpg file size, not pixel dimensions?

Question

I am preparing images for the web and I really have 2 questions: one about gifs, and one about jpgs.
My standard procedure is to reduce the image to the desired pixel dimensions at 600 dpi
That gives me a crisp small image. then I either use it as is if the file size is low enough (I try for under 600 kb) or convert it to a gif with the save for web and devices tool.
So here are my 2 questions (I will count this solved with either answer)
1) When I convert to a gif I have the 4 boxes: one with original size, the other 3 with options but often the options are too low res for me How do I change my 3 options to start at a higher gif res?
2) If I try to reduce the file size of the jpg in the image size box I set the resolution lower ( 400, 300), which lowers the pixel dimensions and the filesize, but I don't want to cahnge the pixel dimensions. And  If I reset the pixel dimensions back to the size I want them, even though it is a lower resolution the file size doesn't change. How to reduce jpg file size using only the resolution, not pixel dimensions?  PS I have tried messing with checking and unchecking the 3 little boxes( scale styles, constrain proportions, and resample) but nothing has worked.

Answer 1

Gif is not a great filetype, especially if you want a crisp image. What are your pixel dimensions?
For web, it makes no difference what the dpi is, only the final pixel dimensions so once you have that as you want, use 'Save for Web and Devices' and you can lower the quality - 100% being max and 0% being lowest. I wouldn't save anything below 55%. For a 600kb image, your dimensions must be rather big.