Here's my 2 cents worth:
1) For the spider shot, the
review shows that the 18-55 is pretty soft at 55mm f/5.6. I guess you could have tried 1/15 f/11 if you wanted more sharpness across the whole frame. With the IS offering 3-stop advantage, handheld shot might be steady enough at 1/15 (depending on how steady your hand is). If you've wanted the spider to be in focus, you could have focus lock on the spider before re-composing you photo. Alternatively, choose another AF point (of the 9 AF) which the spider is on.
2) For the tiger shot, this
review shows that @ 135mm the 55-250 lens sharpness peaked at f/8.0-11. With f/8.0-11, you shutter speed would be around 1/320-1/160 and you picture will have less problem with subject movement or hand shake. Try to avoid going beyond f/16 as diffraction could degrade the sharpness.
3) For the elephant shot, the 18-55 should be quite sharp @ 18mm across frame (sharpest in the centre). However, at f/3.5 the Depth of field (DOF) is quite shallow. If you've wanted the whole face of the elephant, including the trunk to be in focus, you could have increase the aperture to f/5.6-8.0. With f/5.6-8.0, the shutter speed may drop to 1/10-1/20, you might want to up the ISO to 1600 so as to increase the shutter speed to 1/20-1/40, with the IS turned on to prevent hand shake. Best is to take a few shots @ f/5.6-8.0 with both ISO 800 and ISO 1600 and choose the sharpest image when you review the images on PC. Bearing in mind that higher ISO will produce more noise, thus reducing sharpness.
I always choose my own AF point rather than leaving it for the camera to decide. By choosing my own AF point, the photos usually turns out closer to what I have in mind. And since the memory card is already paid for, I almost always shot in continuous mode with AF set to AI SERVO. It always better to take a few more shots and choose the sharpest than to return home without sufficiently sharp images.