You can enable a retention policy in SPD which will do what you want. When you get to the options screen of the backup wizard, click on Advanced. From there click on the Retention tab. Tick to enable the retention policy and then specify how many images you want SPD to retain and also if, when deleting old images, you want SPD to delete just the incrementals or the full image.
If you have already set up your backup job, you can apply the retention policy by editing the job (from the SPD window, click the Backup Jobs tab, select the job you want to edit and click the Edit button). This will launch the Backup Wizard for that job. However, it will only apply the retention policy to that point forward, not on images and incrementals created before you applied the retention policy.
Regards
Charlie