If you have 250 files, consider naming by number 001 to 999. If you had 1000 photos, then you would consider 0001 to 9999.
The way to get files imported into Premiere Elements in the preferred sequential order is to order the photos in a folder on the computer hard drive.
Then import the whole folder into Premiere Elements 12/12.1 Expert workspace Project Assets. Then from Project Assets drag the whole folder into the Timeline Video Track 1/Audio Track 1. When you do this, the folder will dissolve leaving the ordered photos on the Timeline.
Order the photos in an Elements Organizer 12 Album that you create there and then import that Album into Premiere Elements 12 Editor from the Elements Organizer 12.
Please let us know if you are OK with the above information.
I couldn't get the numbers to be changed on the image files themselves. So I set up three folder in Project Assets for files labelled as follows: 0-9, 10-99 and 100-250. That stopped missing them up. Thanks.
Thanks for the follow up with the good news that you have your files ordered as you want on the Premiere Elements Timeline.
Please let me know if we need to discuss considerations for doing 0 to 250 in one folder instead of 3 so that the Timeline content order is as wanted. But you are moving forward in your Premiere Elements project, and that is excellent. Good job.