Reason why you see this message is due to PHP memory leakage. PHP is trying to load a large image or file into memory and is not finding enough space for it to load.
If you think, that you will need the necessary feature to upload large files then there are two ways by which you can enable this feature.
i) Edit the physical file and have the line
ii) Edit php.ini and add the following line. You need to restart the apache/IIS server once php.ini file is modified.