Also when using IndexedDB, make sure to store images as Blob, not base64. Else your users may experience very bad performance if they use some other add-ons, mostly adblockers: Performance issues due to add-on incompatiblity
One thing to note though, in Firefox you can send blobs using browser.runtime.sendMessage, in Chrome you cannot. But you can still convert them to base64 on the fly if needed or if you just want to display them, you can use URL.createObjectURL.