Firstly, may I ask why you are using such a dated version of the CMS? You will generally receive a better experience if you use the latest version, which is 11 at the time of writing.

The issue you are experiencing is occuring because you do not have a MediaDescriptor registered which supports .PNG files.

See the documentation in relation to this: https://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/75/Content/Assets-and-media/Media-types-and-templates/ - that's specifically tailored to CMS 7.5.

In a nutshell; you need to add the following class:

[ContentType(DisplayName = "Image", GUID = "{INSERT_GUID}")]
[MediaDescriptor(ExtensionString = "jpg,jpeg,jpe,ico,gif,bmp,png,svg")]
public class ImageFile : ImageData
{

}

Ensure you replace the INSERT_GUID text with an actual GUID.