This is a known Adblock problem (not only in Chrome, the problem is also related to Firefox), affecting images of 300x250 size, and also possibly affecting other images of a certain size (at least in the version of Firefox) - see the easylist section below.
The rationale for this is that since 300x250 is the standard ad size, it is automatically blocked by the adblock. You can confirm this by turning off the ad unit - now you should see the image.
I found the easylist here for the version of Firefox , and it shows several types of dimensions that could block the image (such as 300x250 , 468x60 , etc.), and I assume that the version of Chrome will not be too heterogeneous.
The only fix for this problem is to either slightly resize the image, or simply turn off the ad unit.
See Errors Problem 7387 and Problem 7390 for more information.
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