+1 for team Photoshop. - end bias
Basically, when we're generally working with images for web Photoshop is the way to go. It is the industry-standard software to handle raster-type image manipulation (not 3D like AutoCAD/Blender). After Effects is subjective, in most real life cases, motion/animation isn't required (e.g: billboards, signs). Since osu! gfx are presented on a web format, adding subtle animations to a banner or profile picture can enhance the viewing experience, so that's a bonus. On the side note, despite Photoshop being mainstream, other alternatives like paint.net or GIMP are also very capable, so feel free to experiment and see which suits you most. They have the same learning curve and transition between them shouldn't be an issue.
If you have any questions or would like to learn more, you can always ask me or these guys above, I'm sure we're happy to share some advice and experience for you.