In my guide to attaching user-uploaded files to Marketo forms I noted that there really isn't any problem storing images (or other documents) in Marketo, provided file sizes are relatively small; the only problem is in displaying the stored data within the Marketo UI. It's easy to get an uploaded image to display in a prefilled form, in an SFDC VisualForce page, or using the SOAP or REST APIs, but the regular Marketo UI will display the data in its raw form (called a Data URI) within a textarea.
You can copy-and-paste textarea contents into a new tab to show the picture, but it's a little simpler if you use this bookmarklet. Drag the link on that page onto your Bookmarks/Favorites bar. Then, when you're viewing a Lead, click the link to change the textarea into its underlying image (the code just reads the TEXTAREA into an IMG, inserts the IMG, and hides the original TEXTAREA, nothing fancy).