01. Obviously, choose a picture... I took this one.
02. Create the path you want the text to follow. To create the path, I use the line tool  on the 'point to point' mood. Make sure 'Create as vector' is checked!
03. So now that the path is created, my image looks like this. Create a new raster layer on top of it, and make sure that one is checked.
04. Click on the the text tool, and more it across the path you created, until it turns to an A with a rounded line underneath it. Click, write the text you want to show on the picture, and set the text to floating. Click on OK.
05. Last step - Go to the selections menu, and choose Select None. Then go into the layer palette, delete the Vector layer the path is on, and you're done!
Hope that helps you guys, and let me know if it's not working :)
ETA: If you want to create a 'wavy' text, do the same, expect you should use the 'Bezier Curve' mood instead of 'point to point'.