The current Zotero Beta includes a built-in PDF viewer which can (besides normal text annotations) annotate images using a simple selection box. The images are displayed in the entry's note file, but zoteroRoam imports them as

<img alt="" data-attachment-key="76R53MGN" data-annotation="%7B%22uri%22%3A%22http%3A%2F%2Fzotero.org%2Fusers%2F5159618%2Fitems%2F85I48EP8%22%2C%22color%22%3A%22%23ffd400%22%2C%22pageLabel%22%3A%22294%22%2C%22position%22%3A%7B%22pageIndex%22%3A1%2C%22rects%22%3A%5B%5B31.276%2C32.825%2C544.899%2C325.43%5D%5D%7D%2C%22citationItem%22%3A%7B%22uris%22%3A%5B%22http%3A%2F%2Fzotero.org%2Fusers%2F5159618%2Fitems%2F8EVGRL8G%22%5D%2C%22locator%22%3A%22294%22%7D%7D" width="856" height="488">

Support for this would make the workflow perfect as the last missing/time consuming part is to manually copypaste all figures into Roam after reading and importing a paper.