Hey caylan2815,
Welcome to Apple Support Communities.
First, we suggest making sure the MMS and Group messages are enabled in Settings > Messages. Once there, scroll to the section labeled SMS/MMS. Beside MMS Messaging and Group Messaging are both enabled.
If the iPhone storage is almost full, you may experience this issue as well. Another reason this may be happening if there isn't enough iPhone storage to be able to send and/receive MMS messages. You can check your iPhone storage by opening the Settings app > General > iPhone Storage.
If your storage is close to full, you may see some recommendations that may help free up space. This article also provides great information regarding iPhone storage: How to check the storage on your iPhone and iPad.
When High Efficiency format, photos and videos are reduced in size. If Most Compatible is selected, JPEG/H.264 is used. You can check this setting by going to Settings > Camera > Formats. The information in this article may be helpful: Using HEIF or HEVC media on Apple devices. According to the article, having it set to High Efficiency format is recorded.
If you continue to experience the issue, restart your iPhone and test again. Restarting refreshes the device's system software, and has been known to resolve some software related issues. After restarting attempt to send a new MMS. Also, have a contact send you a new MMS. Hope this helps.
If you continue to have issues, or you're experiencing a different issue, providing more details may help you get more answers.
Take care.