This issue might sound funny but recently I have across this issue.
We have a wordpress network on subdirectory with domain mapping plugin installed. When we setup the network, few sites were using the path to images through their own domain and few images were linked via the main domain of the network.
For example if http://www.abcd.com is the main domain of the network and xyz.com is a domain which is mapped like abcd.com/xyz. For the site in xyz.com we linked few images through abcd.com and few images through xyz.com.
This was working perfectly, and we were able to access all the images. But suddenly on one fin day, we were not able to access images linked via abcd.com and with further drilling we found that this is happening with image name having logo.png.....so if we link the image as xyz.com/wp-content/themes/xyz/images/logo.png it is working fine but when we link it like abcd.com/wp-content/themes/xyz.com/images/logo.png it does not show the image.
So can any one help me with the issue. Considering it happned all of a sudden and we are sure we did not make any changes.