The process for inserting images using XDocsFMx is a little different than you might expect.
First, any image you want to add to a document must already exist in the XDocs repository; you should not insert an image that is referenced from the local file system. Use the XDocs Explorer to upload an image file into an appropriate location in the repository.
Inserting a fig or image element will display the Anchored Frame dialog or panel (depending on the FrameMaker version you’re using). Specify the Anchoring Position and Alignment, then choose New Frame. After the frame is inserted, the Browse Files dialog displays where you can locate and select the image to insert. After the image has been inserted, you can resize it by selecting the image (click on the image itself) and choosing. In the Object Properties dialog, set the height, width, or DPI. Changing other properties will have no effect since those properties are not saved in a DITA file.