How to direct the user to download the full-res image because the context-menu downloads the low-res thumbnail
a) Let the user download the low-res preview, but provide clear and visible link to the full resolution image
+-------------------------------+
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
|XXXXXXXXXXX THE IMAGE XXXXXXXXX|
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
+-------------------------------+
[download full resolution]
b) For unexperienced users: Avoid icons and symbols. Be textual, plain, simple and direct.
Icons and symbols are not universally understood. They make the interface harder to learn. For the unexperienced user to understand what you mean, make a link with a written simple expression and highlight the key words.
Very artistic designs can be very bad for productivity and work. Most content oriented sites have a minimalistic interface. Take stackoverflow as an example: there is icons and symbols, but they are not needed to read the answers, navigate and answer a question.
c) You may limit the instructions to appear over the image only when the user selects
You may choose show the download link only when the user is selecting the image. Since it will appear close to where the user is looking it will probably will catch his o hers attention. This may make a cleaner design, but the down side is the user has to select the image to know it can be downloaded in higher resolution.
Normal
+-------------------------------+
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
+-------------------------------+
mouseover | mousedown | selected | touched | focus
+-------------------------------+
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX|
|XXXX Click to download fullres |
+-------------------------------+
Remember that smartphones and tablets doesn's not have "onmouseover" and browsers have different sets of events for mouse and touchscreen as described by Coebergh. Using onmousedown, pointerdown, touchstart, ondragstart and onfocus you probably will catch the user before he or she sees the context menu. But I don't know how stable this is. I never implemented that because in my line of work small screens are not an option.
d) override the context menu
I would advice you not to override the context menu. A standard browsing interface is comfortable to the user. It allows many users to do some actions automatically without thinking due to "muscle memory". But is an option. If you limit only to the image, it may not break the navigation too much.