After you open the image and make any changes you might want (size, contrast, other stuff or nothing), then click on save (file, save). A screen (dialog box) will appear to the right of the save process with a sliding scale from 0 - 100, along with some other options. Choosing 100% will keep the picture the same size in KB. If you choose 50%, it will be much smaller in size.

I usually resize the picture (image, resize) to about 640 * 480, then save at about 85%, which should make it plenty small to upload, yet still retain some quality.