How can I save the image as a high-resolution large file size JPG?
Always use a lossless format such as TIFF or PSD to save your master files.
You should only ever save a file in Jpeg format once you are sure it will never need to be reopened and resaved. Each "save" discards pixel data which can never be retrieved, and will progressively impair your image quality.
Paul, is 2.5 MB the size of the original .jpg file itself on the disk, or is it the size of the image reported by Photoshop when it’s open? If the latter, the change in file size would be expected. For example, if I take a large 8-bit RGB image and reduce the image size so the data is 2.5 MB, it is 1152 x 768 px in size, or about 0.9 Megapixels, which is not very high resolution. Saving this in JPEG format, the file size becomes about 250 KB with medium-high JPEG "quality" settings (59 in Save for Web & Devices, 7-8 in Save As without profile attached). The quality settings simply tell the JPEG compression routine how badly to mangle the pixels in trading off accuracy against file size. Saving a low-resolution image at a high JPEG quality setting won’t improve its resolution.
On the other hand, a 2.5 MB .jpg file will be a much larger and higher-resolution image. For example, the image file I used above is 4525 x 2977 px, and is a 38.5 MB image when open in PS. If, instead of reducing the file above to 2.5 MB of image data as above, I simply try to save it as a JPEG using Save As, the resulting .jpg file will be 2.5 MB at quality setting 9, but I can’t get it down to 250 KB even at a quality setting of 0.
