Based on the info you gave me, here's what I was able to figure out:
It is a 40 byte header.  I actually believe it's a 10 uint32 header.  Based on that, this is what I believe is the record format of an image:

Position	Purpose
0	?? - File Type? Always set to 0x8005 to indicate it's an image?
4	?? - Unknown Value
8	?? - Unknown Value
12	?? - Unknown Value
16	Width of image
20	Height of image
24	?? - Unknown Value
28	?? - Unknown Value
32	?? - Unknown Value
36	The pixel format used in this image
40	The actual pixel data as an array of int16 (`Width` * `Height` * 2)

I have the data file reader processing a RGBA4444 format into a class called Image.  Each pixel in the data is converted from uint16 to ImagePixel class that contains Red, Green, Blue, Alpha properties.  That was simple since and just need to do RGBA565 next.  I'll output some of these as PNG just so I can confirm they are correct.  Honestly, I don't think I'm keeping ImagePixel as it's a waste of space since we're not doing direct image manipulation and uint32 works just as well. 

The other thing I noticed is that all images are 256 pixels wide and the images basically wrap.  I'm sure one of those values in the header describes the wrapping, but haven't really analyzed multiple records yet.

Blah,

I literally had a whole bunch of info typed up then my session timed out... Going to get some things put in word, then out on git (code you have out there now looks great, btw!).

But pretty much, here is some quick info on the image formats used in blitz (the ones I know of).

The Images are the same images used in the NFL Blitz PC versions (99 ans 2000).

Here are the PC images:
http://www.mediafire.com/file/wzgreo0am0kvbyt/data.7z/file

The headers are 41 bytes from what I can tell.
There are 2 images formats:
RGB565
RGBA4444

offset 36 (decimal) tells you what image type the file is:
        rgb565 = 0x0A,
        rgba4444 = 0x0C

My idea was to make a console app to look up each image file in the arcade raw drive and record its offset. Open to better ideas though.

Hey jaker,

My physical copy of NFL Blitz PC came today so I'm all set. I'll try to get a copy of NFL Blitz 2000 for PC someday too. Good luck with your work, I can't wait to give the PC version some roster updates like we've given the N64 version Have you thought about expanding the file editor to work with PC files? Custom textures on PC would be a cool next step to look into for PC Blitz hacking.

Sounds good! I'm sure the repos you saw are mine haha. Currently most of the development I'm doing is in a separate tool. More of a POC to get things working. I'll try and get that out on github this week and send you some instructions on what I'm trying to do, for sure would love any help I can get

Thanks!

Hey jaker,

It's me, Lugia. I wanted to change my username but found out I couldn't so I decided to create a new account. Apologies for any confusion. If you need someone to test stuff you're developing for PC Blitz I can do that. I bought a copy of it off eBay today and it should be here by late next week. If you make any breakthroughs on hacking PS1 Blitz I can help with testing that too.

I have an ISO backup of PC Blitz as well that I downloaded from ISOZone a few years ago before that got shut down so if you want anything tested before my physical copy arrives I can test stuff with that. Are you working on PC Blitz or 2000? I think some cracked versions of 2000 are floating around online but I haven't tried those. PC Blitz works fine on my laptop but I remember having trouble getting 2000 to run on Windows 10 in the past.