in the File includes/functions/html_output.php in the function tep_image there is a check if the image size should be calculated:

// Do we calculate the image size?
if (CONFIG_CALCULATE_IMAGE_SIZE [...] )

This check is incorrect because in the admininterface the CONFIG_CALCULATE_IMAGE_SIZE is set to "true" or "false"(both strings!) and therefor this part of the check is everytime valid!

it should read:
if(CONFIG_CALCULATE_IMAGE_SIZE == "true" [...]

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