Good inputs by @JeffK
I have a sample with Layers below on one approach.
Yet, I think you would be best off starting with a higher resolution image of the turtle. If you just upsize the turtle to fit the background, you may find the turtle is no longer sharp or is pixelated. In my example, I upsized using Topaz Gigapixel for the example yet hope there is just a high resolution version available to you.
I made a Layer mask around the Turtle only (you may want to mask it differently)
Then used File > Place Embedded.......with the background image underneath and scale and position as you see fit. (same as JeffK suggested as it turns into a Smart Object with this approach
This way, you don't lose much of the background detail as you don't resize very much and by starting with a higher resolution turtle, it will look sharper as well
Hope these additions to JeffK's post helps
First, my apologies. I misread your post and thought you were concerned about the background.
In any case, I went back to the turtle, roughly masked it, and tried the downsizing then upsizing of the image as explained in the link I provided.
Here are the side by side results - ie original image resize and then taking the second image, converting to smart object, and resizing:
Pretty dramatic - even I was taken aback by it.
I don't know what size image you want to end up with when you're done. From what I can see, your background is 3500 X 3500 while the turtle image (unmasked) is 376 X 279. If you wanted to upsize the turtle image to the background, that would be a 10X increase.
If you take the existing turtle image mask, convert to smart object, place on background, here's what you would get:
Here;s a closeup of a section:
You end up with a very fuzzy turtle but a reasonable sharp background.
As @thebestcpu (John) wrote above,it would be best to start off with a higher res / larger version of the turtle. If this is the only copy of the turtle you have, you may want to consider a smaller reproduction/final size. At least you would you have a better visual.
Here's on at half size of the above - still fuzzy but visually a bit sharper:
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