fails upload images in posts + wp-admin/upload.php 403 error

To be able to help, some info is needed: Is this a production site, or a test one? Are there many plugins? Most wp problems come out of plugin failures or misconfig.

Having said that,

  1. Do you upload correct size of images? Remember that uploading too large images is pointless, as it increases load time and these images are scaled by the browser anyway – so they don’t look well if they are too big.
  2. Do you have set proper memory allocation in wp-config.php and .htaccess files? Are both values the same (sometimes this error is because of value difference)?
  3. Do you have cache or website image processing plugins? Purge cache, disable these plugins and check if error appears.
  4. When did you start to get this error? Was it after update or a plugin installation? If so, turn off plugins one by one and check if error appears.
  5. If above doesn’t work, enable debug mode in wp-config.php