1. Click on New Image
2. Fill in the Subject and Description for post
3. Click on Image Manager
4. Click on Browse
5. Select image from your computer
6. Write description
7. Click on Add File
8. Repeat 3-7 until 4 images or less have been uploaded, then click submit

Uploaded images at 640x480 pixels which seem to work well. Note the consistent thumbnail sizes. As long as your images are a consistent size, you will get a more uniformed look. As for editing a Gallery post, you can only edit the text in the post, not the images i.e. replace or remove images.

Have fun!