This is perhaps overkill, and would only help going forward rather than with existing images already shared in Slack, but...
One trick that could work is to automatically save every new image and other file shared in Slack to a Dropbox or Google Drive folder, then use your computer's file viewer and search to look through your files normally. I've set this up before with Zapier; have Zapier watch Slack for new files, then save them to Dropbox, Google Drive, or other cloud storage files. It gives you a backup of all your files, and makes it easier to search, preview, and use the files you need.
Another option might be Command E, a new beta search tool that works with Slack. @JasonYanowitz, have you used Command E to search through files in Slack, and if so, what does its preview look like?
Especially for personal to-dos and tasks that you need to accomplish outside of your team, what is your favorite app and tricks for getting the most out of it?
Last week, Slack announced some new features, including a WYSIWYG rich text editor.
Along with the addition of this feature, they changed the Markdown functionality to render formatting in-line.