I just checked my old Droid and my Bionic. Astro works the same. If you choose the Astro Image Viewer, you can't zoom. In the Droid, if you choose Gallery as the viewer in Astro, you can zoom. In the Bionic, if you choose View Picture, you can zoom. If you had previously set the default as Astro Image Viewer, you can go to Settings > Applications > Manage applications > Astro > Clear defaults. The next time you view from Astro, you can choose View Picture as the default, or leave the default unchecked so you can choose either.