I've made a bunch of images and they're very unique and I wanted to have my own little watermark image in the corner, like a transparent image. How would I do that? Thanks.
Look at mine in my profile. Just create your image on a transparent layer using black, about 500x500 and then create a brush with it. To apply it to your images, choose white as the foreground color, pick the brush you created and resize it accordingly, then create a new layer on top of your image and hit it once or twice with your brush, depending on how visible you want it. You'll find a link on creating custom brushes HERE.
I was thinking more like that diagonal text you have in your tutorial there, that says 'fragger' in the background.
I know how to slant text but how would I make it repeat over and over like that?
Well you could do it with a brush as Fragger suggested.
Create your text on a clear background Edit>Define Brush
Then paint where you want, you need to remember to save it though.
Or you can do Create your text on a clear background Edit>Define Pattern Then Edit>Fill>Pattern
I did the diagonal text as a pattern, as S3ph said.