I understand that the rules can be somewhat difficult at times so hopefully this post can help clear up any confusion you may have!
guidelines
Summary: Don't add anything that isn't already in the PSD. Play with what is!
You can always manipulate the base image layer for the fullsize image. CTRL+T (or ⌘+T if you are on a Mac) - handles should appear at the corners of the image. If you hold down Shift, then click & drag one of the handles inward, it should make the image smaller.
If using a layer mask, you are allowed to use the brush tool (Basic Brushes only), but ONLY to remove, not add.
For example: you can use the brush tool on a layer mask to remove part of the layer (like you would an eraser), but you can't use it to draw on top of an existing layer.
Please only use the Basic Brushes when using the brush/eraser Tool -- preferably just the circle brush in different widths.
If there is text included, please DO change the text/font/etc on the text layer. Do not, however, ADD text.
examples
When in doubt, ask! As always, if you have any additional comments or concerns regarding a specific action, or whether or not something is allowed, feel free to leave a comment and I will do my best to get back to you! Chances are others have had the same question, so look through everything before leaving a new comment, and I'll also try my best to update this post with any other specifications.