Please help! I have been trying to post photos of a rifle for a wanted to sell ad but it keeps telling me it's too large of a file. What's the secret here? I do not have a desktop btw, just my android smart phone where said pictures are in my Google Photos. Thank You

There are many photo resize/compression apps in the apps store. I have not personally used any of them (I use a desktop for photo editing work), so cannot recommend any one over another.

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The limitation is file size, not resolution, so if you can compress the file size down to 500 KB or less, it should work as an attachment (do note you dont want it to get TOO small, as reducing the size also reduces the details it can show, and making it too small will make it fuzzy or blocky) 

I am not sure if there is a limit on number, if you do hit a limit on the total number in a single post, you can post a reply with the remaining attachments.

There is a size limit of 500 KB per file. If your photos are larger than this, you will need to use a file resize or compression tool to shrink them down small enough to post.