Let x = the number of sheets of four 3x5's
Let y = the number of sheets of two sx6's

So, since the x sheets will give 4x pictures,
and the y sheets will give 2y pictures, both
together will give 4x + 2y pictures.

Also, since

>>......Each sheet of printed pictures costs $8,......<<

and there are x + y sheets, the total cost is 8(x + y)

>>......You want at least 16 pictures of any size......<<

So

4x + 2y > 16

>>......you are willing to spend up to $48......<<

So since total cost = 8(x + y),

8(x + y) < 48 

You also know that the number of sheets cannot be a
negative number but can be 0, you have these two
inequalities:

x > 0 and y > 0

So here is the system of linear inequalities which
models the situation:

4x + 2y > 16
8(x + y) < 48
x > 0
y > 0

Edwin