Problem with Textra MMS

300kb isn't really very big if you're sending a photo.
A quick check in my pics directory shows that mine average about 3700kb, with smaller ones coming in at over 700kb.
It might simply be that you need to take lower res pictures for this to work (this all assumes that you're sending pictures via MMS). If I'm not mistaken, the Samsung app auto-resizes photos on the fly before sending them via MMS, which is why you're not seeing the problem there.

Go into the options and make sure the file size is under 1mb. I had a similar problem and that fixed it.

If you have the message that says Picture too large while using Textra, read below.

The REAL way to fix this problem is very simple. I never had issues sending pictures until today and I know EXACTLY WHY and I fixed it.

I was messing around with my Uncle making recordings and sending them but anything close to 1 minute is too large to send. I went into MMS settings in Textra and changed the limit from 600KB to No Limit. Yours may be 1MB by default but mine was 600KB. This allowed me to send long recordings to him. However for some reason this changed my picture messaging. Although I had it set to NO LIMIT for some reason Textra would say picture too large which makes no sense because no limit is no limit. I changed my messaging app back to default which worked but won't work in Textra.

After thinking about it I said to myself you know what let's put the limit size from no limit back to 600KB but I set it to 1MB instead and now textra sends pictures with no fails saying picture too large. It's weird to me that no limit would have issues but restricting it fixed it. That's really odd to me. Go into Textra, go to settings, scroll down to the MMS section and press MMS Settings. In there under general the first one will be Send Size. If for some reason it's set to no limit then that's your problem. Simply change it to 1MB and it should allow you to send pictures through Textra.

Also, if you would like a little noise when your text sends successfully go to Settings, scroll down to the 2nd section which is Sending, and tick off the last box that says Play Sent Sound. I personally really like this because I'll know if my message sends. You will hear a little woosh sound. The stock messaging app doesn't have this feature, and I think it's a great feature. This way if you happen to be in a building or elevator and you send a message you won't have to look to see if it sends or not because you'll hear a little sound which is different than any notification sound you have as this sound is a unique sound and can't be changed.