The following only applies to adding photos using a PC. I don't k now about Macs or smart phones.
The simplest way to upload photos is "Switch to Full Reply Screen" which is below the Quick Reply text screen that you typically type your response in.
Then, click to "File Manager"...A pop up window will appear.
Then, I use the Browse Button to go out to my computer drive and find my photo. MAKE SURE YOUR PHOTO SIZE IS LESS THAN 2MB.
(I use IRfanView free software to resize photos among other things - This resizing exercise can be done a million different ways and which way is up to you.)
Once you picked your photo you will be directed back to the pop up window and you must click the "Add file" button.
If that is the only photo you wish to post...click "Done adding files".
If you wish to add more...use the "Browse" button again until you have selected all the photos you want to post, then hit the "Done adding files" button.
You will be taken back to your post and the "File Manager" option below the text box should indicate how many "total Files" you will be uploading.
Then, the attached photo will look like this in your post...
