Added type annotations to image_mobject.py

[henrikmidtiby]

This PR adds type annotations to the file image_mobject.py

Further Information and Comments

When running mypy I get this error

Signature of "set_color" incompatible with supertype "manim.mobject.mobject.Mobject"  [override]

This occurs in two locations, around line 70 and line 227.
The issue is that the set_color method implemented in this file also have the named parameter alpha which is not present in the parent Mobject class.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested