There can sometimes be issues with the built in PDF viewer, which may cause fonts or TikZ drawings to appear incorrectly, and cause some images not to appear (though a more likely cause is that the Recompile mode is set to draft). This can be resolved by changing the PDF viewer from built in to native.
To change the PDF viewer, simply click into the on the Overleaf menu icon above the file list panel:
and scroll down to the PDF viewer option
Note that the SyncTeX buttons (jump-to-preview and jump-to-source) will not work in Native view. Each recompile would also return to the first page of the output PDF in Native view.