I would like to integrate the post above by Henry Goh.
The 48 MB requirement is tipycal of Alamy so I also suppose it's Alamy we are talking about.
This 48 MB thing "uncompressed" has caused much misunderstanding and is actually an ambiguous way to describe size image. You might save your JPEGs as TIFFs and they will have a dimension in Photoshop (let's say 48 MB or so) then you go to Windows Explorer and you see they are smaller (evidently consensus on what a MB is is still to come).
So the basic rule in Alamy is this:
You can send JPEG file, provided that the pixel count of the actual image (once opened in Photoshop in 8-bit depth) is at least 16 millions. So you just multiply the height in pixels for the width in pixels, and if the number you obtain is > 16,000,000 then the image satisfies Alamy's minimum size requirement.
(Note that I say "million pixels" to avoid the use of the expression "megapixels" which would probably raise the question whether a megapixel is 1,000,000 pixels or 1024 x 1024 pixels).
Your image can be bigger than this. I normally send images around 55 MB (scanned).
If the dimension in 8-bit format is less than 16 millions pixels than you upsize it just to be above 16 millions pixels. As you are interpolating there is no need to go beyond the minimum requirement.
So your normal workflow from your JPEGs would be.
a) Open them in Photoshop;
Convert them to 16-bit TIFFs;
c) Do all the corrections you want to apply to the file;
d) Check pixel dimensions;
d2) If pixel dimension is not OK, scale up;
e) Convert to 8 bit file;
f) Save as JPEG maximum quality.
This JPEG file is what you upload to Alamy. Its maximum dimension must be 20 MB you don't have a problem of exceeding maximum dimension if you start from a DSLR.
Your JPEGs images might fail Quality Control if there are JPEGs artifacts visible in the image (patchworks of squares 8 bit size for instance, or diagonal lines being jagged, you can spot those problems by inspecting the image at "actual pixel size"). Conversion to JPEG format which is made "in camera" might not lead to results acceptable to Alamy (also depending on your camera settings).
As far as I can grasp from Alamy forum, images captured with JPEG "in camera" are not much beloved by Quality Control. Your mileage may obviously vary.