API v2: Simple image gallery on my website using dropbox

Apologies, I wasn't clear before. Even if you don't specify the format, the returned data is already the jpeg data for the thumbnail for the file. In either case though, that is, for png or jpeg thumbnails, the thumbnail data will be returned directly in the response of the API call as binary data.

It sounds like you want to display these thumbnails directly in an img tag, in which case you will need to base64 encode it for a data URI as you showed, or save it to your server and serve it from a URL you can use in the img src.

I'm not sure what the issue may be when you say that when you "display the image in a modal box the image opens in a new window". That sounds like it may be outside the scope of the Dropbox API itself though, since the API call already succeeded and returned the thumbnail data at that point.

Perhaps you'd prefer an API endpoint that returns thumbnail links, instead of thumbnail data? I'll send that along as a feature request too.