I´m looking to switch to an iMac. Can I download Adobe Photoshop on it or is there a compatibility problem?

Mostly it depends on how your going to be running it. If your using a PC format like windows and that is the photoshop you have then that is your native OS.

If you want to use the Mac OS to run it then its not going to work. You will need to get the Mac version of the software.

Adobe - Photoshop : For Macintosh (http://www.adobe.com/support/downloads/product.jsp?product=39&platform=Macintosh)

Photoshop has versions for both Windows and MAC OS.

And you don't download software to a computer. You install it. You may download the install files, but that is different.

Photoshop is an expensive package.