@Joria - I have just checked a couple of your pictures and they are jpgs, not pngs, so you should be alright to continue posting your stories and creations. I did just check ones in your runaway teen story, not your Sims 4 ID/Creations threads, so you may want to check those as well if you are concerned.
For anyone concerned about your images being of the incorrect type, it is a simple process to check image types! Here's how I do it:
1) Go to the post that has the image you want to check.
2) Right click the image you want to check.
3) Select Open Image, View Photo Info or Properties from the drop down. (Depending on what browser you have.)
4) In the pop-up box that comes up - look for the word Type: If this says PNG Image, then your image was saved in .png format and will need to be changed to a .jpg format.
5) Repeat the above 4 steps for a few more pictures in one of your posts to ensure that it wasn't a one off jpg file, but provided you save the pictures in the same way each time, they should be the same format. (Note the SHOULD, not WILL in that last sentence)
A disclaimer: I am using Google Chrome on a Windows 10 operating system on a PC. I believe that the process should be the same as above if you are using a different browser, operating system or a Mac, however that would need to be checked to be sure.
@Carl @MrsFlynn If the above steps should be useful, please feel free to add them to the first post in this thread/other threads.