Highlighting on hovering is worth doing, but, as you discovered, it’s not adequate by itself since the user has to make an effort to “feel” the interface to see what it can do, rather than just look at it. Hover effects also don’t do much for tablet users.
If you’re sure the problem is the non-rectangular shape, then work with the visual design to combine rectangularity and arrow-ness in a single acceptable shape. One alternative is simply:
Another closer to your original “paned” design is
If the problem isn’t so much the non-rectangular shape, or you can’t come up with any visually acceptable solutions that incorporate rectangles, then you can try incorporating static borders around your clickable images. A “raised” border will give the images a button-like depressable look that may encourage clicking. This is actually preferred if the “links” actually initialize an action (like “acquire kitty”) rather than simply display information. So, something like:
There’s also the old standard to outline clickable images with the same color you use for links. It might still work, especially if your link color is a shade of blue. From seeing text links on the page (e.g. in the "typical navigation" block), users may generalize that the use of the same color means the shapes are links too. So it could look something like:
If no graphic solution is acceptable, you can include text with the images (in or under them), and make them visually links (colored and underlined). Text could be a label for what the link does or links to, which may be necessary as often pictures are not sufficiently self-explanatory. Worse case, use a generic label like “Step 1.” Even a simple instruction like “(click)” would help, although it’s rather klugey.
At first users will just click the text, which at least keeps them from getting stuck. Once they click the first time, they’ll see the whole image light up, which will signal them to click the (easier to click) image the next time.
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