1. An image acquisition apparatus which electronically records an image of a subject, comprising: an image sensor configured to shoot images with resolutions of the images being switched; a resolution switching unit configured to switch the resolutions of the images to be output from the image sensor; an image selecting unit configured to select a basic image from a plurality of output images which are output from the image sensor, wherein the plurality of output images have different resolutions of at least two types switched by the resolution switching unit, and include at least an image having a first resolution and an image having a second resolution higher than the first resolution; an image displacement estimating unit configured to estimate a displacement between the basic image and another image; and a resolution converting unit configured to convert the resolution of any image in the plurality of output images output from the image sensor using the image having the first resolution and the image having the second resolution shot by the image sensor and the displacement between images obtained by the image displacement estimating unit.