Some of the webhosting providers have installed CPanel webhosting control panel and pyxsoft antimalware protection that can couse the problems with uploading images or php files through your php CMS, or other custom php uploader. The qustion is how to fix this problem.
Fastest solutin for this is to turn off your mod_security module that you have in CPanel if your host enable it for you to manage it. You have to log in to your cpanel and find under Security part ModSecurity, on my panel it look like this:
So when you click on that ModSecurity option you will find option to Enable or Disable mod security for your domain or domains if you have:
So if you disable that mod you will be able to upload your files. The reason you can tupload them with Mod Security enabled is becouse pyxsoft antimalware is configured to block any unwanted uploads from any hosts, usually hackers upload exploits/scripts with .jpg or .php extension, so antymalware will block any unwanted uploads and in some cases block IP of uploader if you try more than 5 or different configured times. Pyxsoft antimalware works with mod security and that is how it monitors uploads. Thats why we need to disable mod security to be able to upload files, or you can contact administrators to add yor ip to whitelist so you can continue your work.