A good place to start would be trying to curl
or wget
the url (you can find the URL in chrome dev tools).
Posting the result here if it's unsuccessful will also help.
If it is in fact being denied at the request level (which is what Apache would be doing if it was done with an .htaccess file), there are likely headers or cookies that allow it to be requested by the app.
If the image isn't drawn on the page using javascript, you can also just simply turn off javascript, which would allow you to right-click and save the image normally, if the app is preventing access on the client.
Check out the headers & cookies sent when requesting the image, and either use those in curl
or an app like GraphicalHTTPClient.
Edit: here's an image of the chrome dev tools, with the headers for an image showing. You'll find this in the "Network" panel in dev tools. Note that the tools must be open when the image loads.
One side note: .htaccess
files are used by apache, which is just a server, and doesn't necessarily have anything to do with php. It's pretty unlikely that the app is denying from their php code (if the app is even written in php), as serving images via php is very uncommon.