Enable image-mode when displaying images in a new buffer

[Stebalien]

This makes it easy to, e.g., zoom the image (and prevents any attempts to edit it, add text to the buffer, etc.).

[alphapapa]

Thanks. Did you branch this PR from master? I'm getting a weird error when trying to check out the PR's branch:

Fetching Stebalien
fatal: couldn't find remote ref refs/heads/fix/scroll-load
error: Could not fetch Stebalien

[Stebalien]

Yes, but that also shouldn't matter. Are you sure you're not trying to checkout #155 (I deleted that branch)?

[alphapapa]

Thanks.