Not all the images that you upload need to be the same size, but they should all be the same proportions (see previous question). One image might be 350w x 700h and another might be 300w x 600h. This doesn't matter, as long as the proportions are the same (1:2 or whatever you make it) the thumbnail will be made without stretching.
If you took a picture with a new digital camera it's most likely much too big to upload. So you'll need to scale it down. You would scale it after you've cropped it (see previous question). If the image you have is too small you can't scale it up, you'll lose quality if you do. You should instead find a bigger original.
To scale images using Gimp:
1. Right click on it and open it with Gimp. When it opens it probably won't be shown at full size. Look at the bottom of the screen that your picture is in and you'll see what percentage is displaying.
2. Make it 100% so you get a good idea of the actual size.
3. Look up the top of the screen and see the pixel size of the image:
If it's more than about 700px in any one direction, you should consider scaling it down. You can go back to the previous question "The result you're going for" and click on the thumbnails there, then look up the top of the full-sized image at the dimensions to get an idea of how big 700 pixels would be.
4. So, if you want to scale it down, click on image at the top and then click on Scale Image....
A new box will open up that looks like this:
Note the chain links next to the Width and Height. If you click on this you'll see they link then unlink. Leave them linked and you'll only need to put in one dimension, either width or height, and when you click on the next box it will automatically adjust the other proportionally. If you cropped your image first (as in the previous question) you'll want to keep the proportion the same and just scale down.
5. Click on Scale then close and save and you're done.
Before you upload your new image you should confirm that the name is okay. It's best to get into the habit of not having spaces or uppercase letters in file or image names. So "Product 124.JPG" should be changed to "product-124.jpg" or similar before you upload.