max-width: 100%;
}
if you make this something like
.entry-content img {
max-width: auto !important;
}
it will overrule the old rule from your template and hopefully fix your isse :-)
The resizing of images at pikachoose is intentional and actually a thing that will need changing pikachoose's JS completely, and I can not do this for you. I'd reccommend you to use the developer gallery type and then using this code to integrate another slideshow script that does support this.
I will take a look at integrating a similar gallery for a future version.
I included a better fix below.

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