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You can get 50MB file in 16 Bit at 300 PPI by upsizing to 3650 (Long side) X 2427 (Aprx short side).

If you have a 6MP camera and generate a RAW file, make the necessary adjustments and save as a TIFF (16 Bit).  Then bring the Tiff fike up and use Image >Resize in PS Elements.  Male sure PPI is set to 300.  Then change the long side of the image to 3650 (Short side will adjust automaticaly),  you then click on the "Bicubic" button at the bottom of the Resize box; from the dropdown menu, select Bicubic = Smoother.

Resave the image as a tiff 16 Bit.  close the image that is up.  Now use File >Open File and check the file size. Roll you mouse onto the file; the little box opens up and you will see that you have a 50MB file.

Open the file and make adjustments to color saturation (light touch - do not overdo it); then tweak it in Levels/curves.  Now convert to 8 bit (Image>Mode>8 bit.  Also use RGB.  If you want, make only a very slight Unsharp Mask adjustment.  I do not use more than Amount (50); Radius (1.2) and Threshhold (1).

When you save this as a JPG file at 12 (Least amount of compaction), you end up with about a 4.7 MB file.  By saving at 10 or 11, it would be a bit smaller for uploading.

Good luck.

Have you really got such small images trough Alamy QC? (Those are only 25MB 8bit)

The image should be 96MB 16bit (-> 48MB 8bit)
I upsize my 5D files in DPP using the batch function and I type the long side to be 5020px.
I think DPP does a good job when upsizing.

What camera are you using? Are your images sharp? (sharp lens, no camera shake...)

Even if Alamy's rules say "no sharpening", I don't think that apply to raw images. Raw images need some sharpening while converting. In DPP I use sharpness level 3 (scale is 1-10) that seems to be the preset. All my images have passed QC.

I tried DPP and now I got "Interpolation artifacts" instead of "Soft or lacking definition".  I think I will try GF5 cause I am not upgrading my camera any time soon.