Partial solvers: rlord3534 (got all of the mini-rebuses), Aleeeeee (almost got there afterwards)
First, get the interpretation of the 3 images:
The first one wasn't hard:
The second one was slightly harder, but was guessed by some:
u's (plural of the letter 'u')
The third was made up of 4 images:
spring $\to$ Spring
some 'er' $\to$ Summer
gold 'tumn' $\to$ Au 'tumn' (chemistry) $\to$ Autumn
win 'ter' $\to$ Winter
So these 4 images represent:
However, this is not the correct interpretation for this final part, and I did say to some that the arrows were the most important.
Note that we are trying to solve a rebus, and I underlined the importance of this in my hint.
The word ‘rebus’ is quite close to the first image, but with an extra ‘re’.
If we apply this ‘re’ to the next image, we get ‘re[u’s]', which sounds like ‘reuse’.
This might lead to the interpretation of the final image which is ‘cycle’. If we add ‘re’ to it, we get ‘recycle’.
This now gives us:
rebus, reuse, recycle
This is quite similar to the phrase:
reduce reuse recycle (which Aleeeeee managed to spot)
Yet the key to solving this puzzle is:
The word 'reduce' is replaced with 'rebus'.
This perhaps indicates that the solution of the original rebus is ‘reduce’.
But this isn’t sports related, and that’s because:
The original rebus did not have any ‘re’s in it — they were placed there by us in order to help solve the puzzle. The image which would have completed the rebus (the image which should have replaced the ‘bus’), should’ve been ‘duce’, or something which sounded like that in order to complete the pattern.
Therefore the final solution is:
reduce $\to$ duce
Which sounds like:
The sport doesn’t have to be tennis, but it most often is associated with it. Note that the theme is more specifically terminology (words/phrases) used in sport.