Depends on what program you are using.

In Paintshop Pro, do the following:

1.  File

2. Export

3. JPEG Optimizer

4. Ignore the "Wizard" button in the lower left of the next window. Instead adjust the compression dial.  The original image shows in the left window, the resulting image from the compression value you selected shows in the right window.  Magnify the image once if needed, but no more because you are concerned about the overall appearance of the image, not each pixel.

Lower number equals better image quality but a larger file size. Higher number equals reduced image quality but a smaller file size.  Play with the dial until you get your image size under 512 KB   [524288 bytes] while maintaining the best possible image quality that you can. The perfect balance is if you get the compression number to give you a file size under 512 KB with no visible reduction of image quality.   Between 0 and  20 usually works well. After that loss of quality is usually noticable.

If you notice that you are having to use too high of a compression value and that the image quality is becoming noticably reduced,  exit from the jpeg optimizer . You will need to reduce the dimenstional  size of your image.  But don't do it through "Resizing"  because the image quality will often be reduced and using bicubic resampling and sharpening doesn't always preserve the quality. Not to mention you will be double compressing it which will result in even poorer quality.

 Instead do the following to reduce the size of the image:

A)  Selections / Select All
B) Selections / Modify / Contract = X number of pixels IE: 10 (equals 20 pixels in both height and width)
C) Image / Crop to Selection

Then go back and do the JPEG Optimizer again (again ignore the Wizard button).

Once you have your file size 512 KB or less and have maintained a good image quality, save the image.

You can do the same thing in Photoshop but you have to go to "File" and "Save to Web" and it's pretty much the same thing.

As for saving a thumnail...

1. Create a new blank image 200 x 200 pixels
2. Select your gallery image and "Copy"
3. Select your blank thumbnail image and "Paste" the gallery image into it as a "New Layer". It will be much too big, but don't worry about the parts that are not showing, they will be removed when you save the image later.
4. Move the image around inside the thumbnail image until you get a preview that you like.
5. Save it using the same technique as above  in order to get the file size less than 15 KB   [15360 bytes].

You will always end up with crisp clear gallery images and thumbs.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi